Low molecular weight polymers are known to be useful as dispersants, scale-inhibitors, detergent additives, chelating agents and the like. A difficulty encountered in the production of low molecular weight polymers by free-radical polymerization is effective control over the degree of polymerization and the resulting molecular weight of the polymers. Various attempts at achieving an acceptable level of control have met with some success, but they all have significant drawbacks. These attempts include the use of chain transfer agents, increasing the level of initiator and using large amounts of metal salts as polymerization moderators.
The use of chain transfer agents has several drawbacks. This approach will impart the functionality of the chain transfer agent into the polymer chain. This can be undesirable since functionality has an increasing effect on the properties of the polymer as molecular weight decreases. Furthermore, the chain transfer agents commonly employed are mercaptans. These materials are expensive and have objectionable odors associated with them. Other common chain transfer agents are hypophosphites and alcohols. These also add to the cost of the process, impart functionality to the polymer, and may necessitate a product separation step.
Attempting to control molecular weight by increasing the amount of initiator is also problematic. This approach adds considerably to the cost of production and may result in polymer chain degradation, crosslinking, and high levels of unreacted initiator remaining in the product. The same is true for chain stopping agents such as sodium metabisulfite. Among the preferred free-radical initiators for aqueous polymerization is hydrogen peroxide. It is relatively inexpensive, it has low toxicity, and does not produce detrimental salt by-products. However, hydrogen peroxide does not decompose efficiently at conventional polymerization temperatures, and large amounts must be used to generate enough radicals to carry out a polymerization.
High levels of metal ions have also been tried as a means for controlling molecular weight. This approach also suffers from increased cost and has associated with it serious health and environmental concerns. In addition, the product is usually discolored by the metal ions.
U.S. Pat. No. 4,314,044 (Hughes et al.) teaches the use of high levels of various metal salts, including copper and cobalt salts, as part of a catalyst system as a means of controlling the molecular weight of water-soluble polymers. This patent requires a molar ratio of initiator to metal salt activator of from 10:1 to 150:1 wherein the initiator is present from 0.5 to 35 percent by weight based on the weight of monomers present. In particular, the metal ions are taught to activate the initiator.
U.S. Pat. No. 4,659,793 (Yang) teaches enhanced incorporation of monoethylenically unsaturated dicarboxylic acids when said dicarboxylic acids are copolymerized with a-.beta. ethylenically unsaturated monomer by using any of several metal salts, including copper and cobalt, and conducting the polymerization at a pH of from 2 to 4 when the level of dicarboxylic acid is from 20 to 25 percent by weight of the monomer mixture, and at a pH of from 4 to 6 when the level of dicarboxylic acid is from 40 to 50 percent by weight of the monomer mixture. The process used by Yang is a heel process wherein the dicarboxylic acid is charged to the kettle prior to the metered addition of the other monomers. Yang only shows the effects of using iron as a molecular weight control agent.
Many boron complexes are known as catalysts and chain terminators in acrylic free-radical polymerizations. However, these boron complexes are generally not water-stable and, thus, are not of use in an aqueous polymerization system. In an article appearing in J. Macromol. Sci.-Chem, A26(8), pp. 1127-1149 (1989), Sanayei et al. discloses the use of cobaloxime boron fluoride as a chain transfer agent in the free-radical polymerization of methyl methacrylate. U.S. Pat. No. 4,837,326 (Lin, et al.) describes cobalt(II) nitrilomethylidyne dipyrrole complexes for use as chain transfer agents in acrylic polymerizations done in methyl ethyl ketone as the reaction solvent. In J. Poly. Sci. Polym. Chem. Ed., 19, 879 (1981), Enikolopyan et al. discloses the use of cobalt(II) porphyrin complexes as chain transfer agents for the free-radical copolymerization of methacrylic acid and styrene. U.S. Pat. No. 4,886,861 (Janowicz) describes various cobalt complexes for use as chain transfer agents for acrylic and styrenic polymerizations carried out in an organic solvent.